Identity-based encryption system for secure data distribution

ABSTRACT

A system is provided that allows encrypted content to be distributed to users over a communications network. A policy enforcement service may use an identity-based encryption algorithm to generate public parameter information and private keys. Data content may be encrypted prior to distribution using an identity-based encryption engine. The encryption engine may use the public parameter information from the policy service and public key information to encrypt the data. The public key information may be based on policy information that specifies which types of users are allowed to access the data that is encrypted using that public key. A user may obtain a private key for unlocking particular encrypted data by providing a key request to the policy enforcement service that contains the public key. The policy enforcement service may enforce the policies given by the policy information and may provide private keys only to authorized users.

BACKGROUND OF THE INVENTION

This invention relates to encryption, and more particularly, toidentity-based encryption schemes for securely distributing data.

It has become common to store content such as songs and video in digitalform. Cryptographic techniques may be used to help secure such contentfrom unauthorized use.

A number of cryptographic techniques are available for use in encryptingdata. For example, symmetric key techniques have been extensively used.With symmetric key arrangements, a first party encrypts data for asecond party using a symmetric key. The second party decrypts theencrypted data using the same symmetric key. Symmetric-key systemsrequire that the symmetric key be exchanged between the parties involvedin a secure manner.

With public-key cryptographic systems such as the RSA cryptographicsystem, two types of keys are used—public keys and private keys. Datafor a given party may be encrypted using the unique public key of thatparty. Each party has a corresponding private key that is used todecrypt the encrypted data.

Identity-based encryption schemes have also been proposed. Suchidentity-based encryption schemes may use public parameters to encryptdata. These schemes are said to be “identity based,” becauseuser-specific identity information such as a particular user's emailaddress is used as one of the inputs to the encryption algorithm. Eachuser has a unique private key based on the user's identity fordecrypting encrypted data. With this type of scheme, a single set ofpublic parameters (used during the encryption and decryption processes)may be shared by many users.

SUMMARY OF THE INVENTION

In accordance with the present invention, a system is provided fordistributing content securely using an identity-based encryption scheme.In the identity-based encryption algorithm used with this scheme, apublic key based on policy information is used as an input, rather thanactual identity information that is specific to a single user. Thesystem may be used to distribute content such as digitally-encodedmovies or other videos, digitally-encoded songs (e.g., MP3 files) orother suitable audio files, text (e.g., books or magazines), graphics(e.g., digital images), stock sale or banking records, software (e.g.,games and other applications), corporate inventory or supply chain data,or any other suitable content.

A content provider (e.g., a movie studio, record label, or televisionnetwork in the case of media-related content, a brokerage house or bankor other financial institution in the case of financial record content,a corporation producing supply chain data, a software distributor ordeveloper in the case of games and other applications, or any othersuitable institution or party) may generate data (content) to bedistributed. A data packaging service may be used to encrypt the data.

The data that is to be encrypted may have associated attributes such asrating, date, title, owner name, price, etc. Attributes may have anattribute name (e.g., “rating”) and an associated attribute value (e.g.,“R”). The data attributes may be separate from the data (e.g., as withthe rating for a movie) or may be part of the data itself (e.g., in thecase of an XML data structure made up of XML fields that serve as bothdata and attributes). If desired, the data packaging service can placethe attributes into the data structure with the data to be encrypted. Adata structure type (e.g., “movie”) may be associated with the datastructure.

Some or all of the attributes associated with the data to be encryptedmay be used as policy information to regulate access to the data once ithas been encrypted. In general, the particular attributes that areassociated with any given data depend on the type of data involved.

The data packaging service may use an identity-based encryption engine(encryption process) to encrypt the data (e.g., when encrypting thecontent of a data structure). The identity-based encryption engine takesthe data to be encrypted and produces corresponding encrypted data. Atleast two inputs are used by the engine in encrypting the data. Thefirst input is public parameter information, for example, publicparameter information in the form of a set of identity-based-encryptionpublic parameters P and sP as described below in connection with thework of Boneh and Franklin. The second input takes the place of the“identity” used in proposed identity-based encryption algorithms. Thissecond input is referred to herein as a “public key,” rather than an“identity,” because it may be associated with a plurality of users,rather than a single user as would be the case if a unique user-specificidentity such as a user's email address were to be used. Despite thisdistinction between identity-based encryption schemes based on theunique identity of each user and the present approach, the presentapproach is still referred to herein as an “identity-based” encryptionscheme to clearly distinguish it from traditional public-key encryptionapproaches such as the RSA public key approach. The identity-basedencryption engine may encrypt the data using the public parameters andthe public key without communicating with other system databases orcomponents.

The public key in the present identity-based encryption scheme is notassociated with any particular user, but rather is based on genericpolicy information that regulates the access rights of multiple users(e.g., hundreds, thousands, or millions of users). The policyinformation of the public key may, for example, be formed using theattributes of the data. One illustrative way in which to assemble thepublic key from the attributes involves the concatenation of attributenames and values. For example, for digital video, the public key mightbe “secure-video://Name=Matrix;Distributor=Paramount;Date=Aug-2002;Rating=“R.” If desired, the data maybe packaged in an XML data structure. Using an XML-based format tohandle data and attribute information may be advantageous in situationsin which XML-based attribute standards obtain acceptance in theindustry.

Data to be encrypted may be packaged in data structures of differenttypes. For example, one data structure type may be “movie” and anothermay be “song.” The way in which the attributes for particular data to beencrypted are used to form the public keys (e.g., which particularattributes are to be used and their order in any concatenation processthat is to be used) may be specified using data type encryption policyinformation. If the attributes are maintained in an XML record, the datatype encryption policy information may be used to specify which of theXML record entries are used to form the public key and how these entriesare to be used (e.g., the order in which certain entries should beconcatenated, etc.). If desired, the data type encryption policyinformation itself may be stored using an XML format. Moreover, thepublic keys may be created using XML (e.g., the public keys may be inXML format).

The content in the encrypted data may not be accessed without firstdecrypting the data. This may be accomplished only through use of anappropriate private key that corresponds to the public key used toencrypt the data. A user who has obtained given encrypted data (e.g.,directly from the data packaging service or associated distributionservices or from another user in a peer-to-peer transaction) may begranted access to the content in the encrypted data (e.g., by beingprovided with an appropriate private key) through a policy enforcementservice.

The policy enforcement service may use various types of policyinformation in determining whether or not to grant access to a givenuser. For example, global policy information may dictate that no videosof rating R may be released to users who are less than 17 years of age.Policy information may also be used to implement commercial subscriptionrules (e.g., if a user is on the “Spielberg Special Plan,” a private keymay be issued for all movies whose director is Steven Spielberg). Thepolicy information that is used by the policy enforcement service istypically provided to the policy enforcement service by the user in theform of an access request containing the public key (and its includedpolicy information), but global policy information (e.g., informationalready known to the policy enforcement service) need not beretransmitted and may be used to supplement or override the policyinformation provided in the access request. Global policy informationused by the policy enforcement server in regulating user access to datamay be based on prearranged industry standards or governmentregulations, etc. Such policy information need not be provided to thepolicy enforcement service by the user, because it is already in thepossession of the policy enforcement service.

The policy enforcement service may be used to administer subscriptionservices (e.g., to process purchase transactions using credit cards,etc.). A user desiring to obtain a private key to decrypt particularencrypted data may provide suitable authentication information (e.g.,information on the characteristics of the user such as informationidentifying the user, user age information, user account information,user security clearance, user membership status, user gender, usercredit card status, or other suitable user-dependent characteristics).The policy enforcement service may use this information on thecharacteristics of the user in enforcing the policy rules set forth bythe policy information to determine whether the user is authorized toaccess the content of the encrypted data. If the user is authorized, theuser may be provided with the necessary private key (e.g., over a securecommunications path such as a secure sockets layer (SSL) path).

Once the user has obtained the private key, the user may use anidentity-based decryption engine to decrypt the encrypted data andthereby access and use the data in its unencrypted form. Theidentity-based decryption engine may be provided as stand-alone softwareimplemented on the user's equipment or may be built into or work inconjunction with other user software. For example, a media playersuitable for playing back videos and audio files may be implemented onthe user's equipment. The identity-based decryption engine functions maybe incorporated into the media player or may be automatically invoked bythe media player when the user desires to play back or view certaincontent.

An advantage of the present identity-based-encryption contentdistribution approach is that content can be distributed in a securefashion, without needing to manage public-private key pairs for eachcontent recipient. With the present approach, a single public key may beused to specify a policy that governs a large number of users. Privatekeys are generated for each public key, but public keys need only begenerated when different policies are implemented (e.g., when a movie ofa different rating or subscription plan is to be distributed). It is notnecessary to generate a public key or to perform a unique identity-basedencryption process for each individual user.

The private keys that are released to users may, in general, be used todecrypt a number of different encrypted data items(i.e., one private keymay be used to decrypt all the encrypted data items that have beenencrypted using the same public key policy information). However,because private keys are released on a policy-appropriate basis, usersneed only be given private keys that are appropriate for their level ofauthorization, which helps to maintain the integrity of the system. Forexample, a user who obtains a private key to unlock a movie that hasbeen encrypted with a public key that includes a G rating, will not beable to use that private key to unlock R-rated movies (because theprivate key would not match the R-rating based public key used toencrypt such R-rated movies). Any movies that can be decrypted by theuser (in this example) will be at the G rating level. As anotherexample, if a user receives a private key that allows the user todecrypt a rated-R movie, the private key might be used to decryptanother rated R movie. However, because the user has already beenverified as having an appropriate age (e.g., over 17 years) to accessthe first R-rated content, the user is not provided with rights that areout of line with the user's level of access rights. At the same time, itis not necessary to encrypt each piece of data using a completelyseparate user-specific public key, which could make the systemburdensome to administer.

Further features of the invention, its nature and various advantageswill be more apparent from the accompanying drawings and the followingdetailed description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an illustrative identity-based encryption systemfor distributing content to users in accordance with the presentinvention.

FIG. 2 is a flow chart of illustrative steps involved in using thepolicy enforcement service to support encrypted content distributionusing the system of FIG. 1 in accordance with the present invention.

FIG. 3 is a flow chart of illustrative steps involved in using apackaging service and distribution services to encrypt and distributedata structures in accordance with the present invention.

FIG. 4 is a flow chart of illustrative steps involved in allowing a userin the system of FIG. 1 to decrypt encrypted data in accordance with thepresent invention.

FIG. 5 is a diagram of illustrative steps involved in encrypting datastructures in accordance with the present invention.

FIG. 6 is a diagram of an illustrative MP3 file showing how music datacontent and associated data attributes may be packaged into a singledata structure in accordance with the present invention.

FIG. 7 a is a diagram of an illustrative data structure (in XML format)in accordance with the present invention.

FIG. 7 b is a diagram of illustrative data type encryption policyinformation (in XML format) that may be used to determine how to use thedata attributes of the data shown in FIG. 7 a in forming a public keyfor use in encrypting the data of the data structure in accordance withthe present invention.

FIG. 7 c is a diagram of the public key (which may be provided in XMLformat) that may be created from the data structure attributes of FIG. 7a based on the data type encryption policy information of FIG. 7 b inaccordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An illustrative identity-based cryptographic system 10 for distributingcontent to users in accordance with the present invention is shown inFIG. 1. System 10 allows users at various locations to receive secureddata using an identity-based encryption and data distribution scheme.The users in the system may be individuals, organizations, or any othersuitable parties or entities. Users may have associated user devices orequipment 22. Equipment 22 may, for example, include computing equipmentsuch as a personal computers, portable computers, mainframe computers,networked computers or terminals, telecommunications equipment, handheldcomputers or personal digital assistants, or cellular telephones.Multiple users may use the same device. For example, a group of usersmay share the use of a single computer terminal that is connected to ahost computer in a local area network. These are merely illustrativeexamples of the type of platforms that the users of system 10 may use.User equipment 22 may be based on any suitable electronic equipment ifdesired.

The user equipment devices may be interconnected by a communicationsnetwork 12. Network 12 may be, for example, the Internet, a local areanetwork, a wide area network, the public switched telephone network, avirtual private network, a wired network, a wireless network, dedicatedleased lines, a network based on fiber-optic or cable paths or otherwired or wireless paths, or a network formed using any other suitablenetwork technology or a combination of such networks.

Various computing devices may be connected to network 12 to support thefeatures of the identity-based encryption scheme. For example, computingequipment at one or more content providers 14 may be used to store andprovide content. The content that is distributed using system 10 may bemedia (e.g., digital video or audio), business record data (e.g., stocksales data, banking or other financial records, supply chain data,etc.), software (e.g., games or other applications), or any othersuitable data. Content providers 14 (which may also be called dataproviders) may be any suitable entities or enterprises associated withthe content. For example, content providers 14 may be movie studios orrecord labels if the content being distributed involves media contentsuch as movies or songs. Content providers 14 may be banks or otherfinancial institutions when the content being distributed is a stocksales transaction or bank account record. Content providers 14 may besoftware distributors or developers when the data being distributedincludes software.

Although illustrated as individual content providers 14 in FIG. 1, thefunctions of content providers 14 (generating, storing, maintaining, andproviding content) may be distributed across one or more actual entitiesor organizations and one or more installations of suitable computerequipment. For clarity, the operations of such distributed entities andequipment will be discussed primarily in the context of one or moreindividual content providers, as shown in FIG. 1.

In some arrangements, the computing equipment of content providers 14and the other entities of FIG. 1 may be used to provide the functions ofa server in a client-server architecture. For clarity, the presentinvention will sometimes be described in the context of suchserver-based arrangements. This is, however, merely illustrative. Anysuitable computing device arrangement may be used to distributeinformation in system 10 if desired. A typical server-based arrangementmay use one or more computers to provide the functions of a server. Aserver may be formed using a single computer or multiple computers. Ifdesired, the functions of a single server may be provided by computersthat are distributed over a number of different physical locations.

Content providers 14 and the other entities of FIG. 1 may be based on asuitable computing platform such as a server-based platform that isconnected to communications network 12.

Some activities in system 10, such as deciding when to initiate datadistribution or user activities such as requesting access to encrypteddata may involve (or even require) manual intervention.

Other activities (including user activities) in system 10 may beautomated or semiautomated. These activities may take place with littleor no manual intervention. As just one example, a data or contentprovider 14 may be associated with a music or video distribution servicethat desires to use encrypted communications to deliver encrypted moviesor songs to account holding users at user devices 22 over communicationsnetwork 12. The distribution process may be automated so that nooperator intervention will generally be needed at the distributionservice equipment once the system has been properly set up. User receiptof the encrypted content may also be automated.

Data from content providers 14 may be provided to a data packagingservice such as data packaging service 16 (e.g., electronically overcommunications network 12). The data packaging service may encrypt thedata before providing the data to users. If desired, data packagingservice 16 may place the content to be encrypted into a data structure(e.g., if this function has not already been performed by the contentprovider 14). Data attributes may be associated with the data. Forexample, rating attributes may be associated with movie content,supplier identities may be associated with supply chain data, artistsmay be associated with song data, etc. Multiple data attributes may beassociated with the same data item. For example, song data may haveassociated attributes such as date of release, artist, title, recordlabel, genre, rating, etc.

The data packaging service may place the data into a data structure ofany suitable data structure type. For example, video content for a moviethat has been provided by a content provider 14 may be placed into adata structure of type “movie.” The movie data may have, for example,associated attributes such as “rating=R,” “Director=Spielberg,”“Price=$10.00,” etc.

The data packaging service 16 may be implemented using any suitablecomputing equipment (e.g., a personal computer, mainframe, distributedor networked collection of computers, servers, etc.). The data packagingand/or encryption and other functions of data packaging service 16 maybe provided by an encryption service 16 or any other suitable serviceprovider or entity 16. These services may be stand-alone services or maybe integrated into other services and entities. Such encryption servicesare generally referred to herein as being “data packaging services” forclarity.

Service 16 (e.g., a data packaging service 16, encryption service 16, orother suitable stand-alone or integrated service) may have computingequipment that is used to implement an identity-based encryption engine.The identity-based encryption engine may have at least two inputs: afirst input made up of public parameters received from policyenforcement service 20 and a second input (the so-called public key)that is made up of policy information. The public key input takes theplace of the user identity information that is typically used in anidentity-based encryption scheme.

Identity-based encryption schemes can be implemented using a number ofdifferent cryptographic algorithms. One such scheme is based onquadratic residues (see, e.g., “An Identity Based Encryption SchemeBased on Quadratic Residues,” Eighth IMA International Conference onCryptography and Coding, Dec. 2001, Royal Agricultural College,Cirencester, UK, by Clifford Cocks). Another suitable scheme is based onelliptic curves (see, e.g., “Identity-Based Encryption from the WeilPairing,” by Dan Boneh and Matthew Franklin, extended abstract inAdvances in Cryptology—Crypto 2001, Lecture Notes in Computer Science,Vol. 2139, Springer-Verlag, pp. 231–229, August 2001. See alsohttp://eprint.iacr.org/2001/090 by Dan Boneh and Matthew Franklin). Forclarity, aspects of the present invention will sometimes be described inthe context of an identity-based encryption scheme such as the ellipticcurve implementation described by Boneh and Franklin. This is, however,merely illustrative. Any suitable approach for identity-based encryptionmay be used with system 10 if desired.

Regardless of the particular type of identity-based encryption schemethat is used, the encryption scheme generally will use at least twoinputs in addition to the data being encrypted.

The first input is public parameter information (e.g., public parametersP and sP in the work of Boneh and Franklin, where s is the so-called“master secret” being used in the identity-based encryption algorithm).In system 10, the master secret is maintained by policy enforcementservice 20. Service 20 may generate the public parameter information(e.g., parameters P and sP) using the master secret. The publicparameter information may be provided to data packaging service 16 foruse in encrypting the data structure (i.e., for use as a first input tothe data encryption engine).

The second input used by the data encryption engine is the public keythat contains policy information (e.g., the public key containsinformation on the rating of a movie or corresponding information in adifferent form that is nevertheless derived from the rating of themovie, which dictates what the required age is for a user to view themovie). When a traditional identity-based encryption scheme is used tosupport (for example) encrypted messaging between a message sender and amessage receiver, the encryption engine makes use of the receiver'sidentity as an input to the identity-based encryption engine. Incontrast, in the content distribution scheme of system 10, the publickey based on the policy information (called Q) is used as the secondinput. This second input is not specific to any single user.

The first and second inputs to the encryption engine are used by thedata packaging service to encrypt the data (e.g., the content of thedata structure). The encrypted data may then be securely communicated tousers at user devices 22 (e.g., over the Internet). The policyinformation used to encrypt the data (e.g., the required age of the userin the ratings example) may be provided to the user along with the dataif desired (or information on what the appropriate associated policyinformation is may be communicated to the user—e.g., in a code). Theuser may also implicitly know or be expected to know that certain globalpolicies are in effect (e.g., that no users under age 18 may accesscertain types of content, etc.)

The data that is encrypted may have multiple data attributes. Forexample, a movie to be encrypted may have an associated rating dataattribute (e.g., R) and an associated studio data attribute (e.g.,Sony). The policy information that is used as the public key whenencrypting the data may be based on one or more such data attributes. Anadvantage of basing the policy information on multiple data attributesis that this approach allows the data packaging (encryption) service tosecure data for distribution to more finely tailored types of users thanwould be possible using only a single data attribute (although a singledata attribute may be used as the basis of the policy information ifdesired). For example, policy information may be formed using multipledata attributes of a movie such as rating=R and studio=Sony. During userauthentication, the policy enforcement service may use thismultiple-attribute policy information and information on thecharacteristics of a given user who is requesting access to theencrypted data (e.g., age=30 and studio membership status=Sony) todetermine whether or not to provide the requesting user with the privatekey needed to decrypt the encrypted data.

The user may use the policy information received from the data packagingservice (or information received from the data packaging service thatidentifies appropriate applicable policy information) to formulate anaccess request. The access request is used to obtain access permissionto the encrypted content from the policy enforcement service. If theaccess request for access to a particular encrypted data item isgranted, the policy enforcement service may provide the user with anappropriate private key to decrypt that encrypted data item.

If desired, the user may use policy information that has been receivedfrom the data packaging service as the basis for the access request. Forexample, the user may forward the policy information that has beenreceived to the policy enforcement service. As another example, the usermay generate an access request that does not include all of the policyinformation itself (or which may not even include any of the policyinformation), but which includes sufficient information to inform thepolicy enforcement service which public key (and corresponding policy)and which private key correspond to the access request. The accessrequest (or a follow-up communication between the user and the policyenforcement service) may include information on the characteristics ofthe user (e.g., user age, membership status, security clearance, etc.)

Regardless of the particular form and content of the access request, theaccess request preferably contains sufficient information to identifythe private key that is being requested. As a result, the access requestdirects the policy enforcement service to provide a private keycorresponding to the public key that was used to encrypt the data. Thepolicy enforcement service 20 uses the access request to determine whichaccess policy applies to the user and uses this policy in combinationwith information on the characteristics of the user that is provided tothe policy enforcement service (e.g., age information, credit cardstatus, driver's license number, membership status, etc.) to ascertainwhether the user is authorized to access the encrypted data.

If the policy enforcement service determines that the user (e.g., theinformation on the characteristics of the user) satisfies the policyconstraints imposed by the policy and that the user is thereforeauthorized to access the contents of the encrypted data, the policyenforcement service may provide the user with the private key (e.g., sQfor systems of the type described in the work of Boneh and Franklin)that corresponds to the public key (Q) that was used to encrypt thedata. The user may use the private key (e.g., sQ) and an identity-baseddecryption engine implemented on the user's equipment 22 to decrypt theencrypted data and thereby access the desired content. Some policyinformation (e.g., global policy information based on legally-mandatedpolicies) may be so well known by the industry, that it is not necessaryfor the user or other entities to forward this information to policyenforcement service 20. Rather, such policy information may bepre-stored at policy enforcement service 20. The global policyinformation may be used to supplement or override the policy informationrepresented by the public key.

Distribution servers 18 may be used to facilitate the data distributionprocess. For example, distribution services in various countries orlocations within a country may serve as downloading services orintermediate transfer equipment. When providing encrypted content to theusers. The encrypted data may be downloaded from the distributionservices on request by the users or may be pushed to the user (e.g.,using a broadcast arrangement). Encrypted content may also bedistributed using a hardcopy approach (e.g., by supplying encryptedcontent on a DVD or CD disk or other storage media, etc.). These aremerely illustrative examples. Any suitable technique may be used todistribute encrypted data that has been encrypted by the data packagingservice to the users if desired. Moreover, the policy information can beprovided to the user with the encrypted data or may be provided in aseparate communication. Global policy information (e.g., informationsuch as the permitted age of access for videos with various ratings) maybe provided at any time and need not necessarily be retransmitted to theuser or provided by the user to the policy enforcement service 20.

In system 10, the second input to the identity-based encryption engine(Q) is referred to as a public key, rather than an identity, becausethis second input is not specific to an individual user and may be usedto encrypt data that is distributed to many users.

The public key Q may be formed from the attributes associated with thedata. For example, one of the data attributes (e.g., rating=R) may beused as the public key. As another example, multiple data attributes maybe concatenated together to form Q. As an example, the public key Q fora movie might be “secure-video://Name=Matrix;distributor=Paramount;Date=Aug-2002;Rating=“R.”

The data and data attributes may be packaged together in a datastructure. For example, as shown in FIG. 6, an MP3 file data structuremay contain both data (e.g., music data) and associated attributes(e.g., title information, artist information, track information, etc.)

If desired, the data may be an XML data structure containing the content(e.g., the video file) and the attributes of the content (e.g., rating,director, date, price, etc.). Using an XML-based format to handleattribute information may be advantageous in situations in whichXML-based attribute standards obtain acceptance in the industry.

If desired, the way in which the attributes for particular data are usedto form the public key Q for that data may be specified using data typeencryption policy information. If the attributes are maintained in anXML record, the data type encryption policy information may be used tospecify which of the XML record entries are used to form the public keyand how these entries are to be used (e.g., the order in which certainentries should be concatenated, etc.). If desired, the data typeencryption policy information itself may be stored using an XML format.

An example of this type of arrangement is shown in FIGS. 7 a, 7 b, and 7c. As shown in FIG. 7 a, an XML format data structure may include supplychain data such as part number data, quantity data, region data,supplier data, etc. Supply chain data is merely illustrative. Anysuitable type of data may be provided in an XML data structure of thetype shown in FIG. 7 a if desired.

Illustrative data type encryption policy information that is provided inXML format is shown in FIG. 7 b. In the example of FIG. 7 b, the datatype encryption policy information contains XML fields that specify howthe data structure fields of the XML data structure of FIG. 7 a are tobe used to form the policy information. For example, the supplier fieldof FIG. 7 b specifies how the policy requirement “must-be-a-customer-of”is to be applied to the supplier field “company=Delco” of the datastructure of FIG. 7 a. Moreover, the region field of FIG. 7 b specifieshow the policy requirement “must-be-in-region” is to be applied to theregion field “region=US” of FIG. 7 a.

The policy information that results when using the XML data typeencryption policy information to specify how to use the XML datastructure attributes of FIG. 7 a is shown in FIG. 7 c. The policyinformation that is used for the public key may, if desired, be providedusing XML (e.g., the public key may be an XML-format public key).

The identity-based encryption process used in system 10 may involve atwo-step process, in which one of the two steps uses identity-basedencryption and the other of the two steps uses a non-identity-basedapproach such as a symmetric key approach. For example, the content ofthe data (e.g., the content of a data structure) need not be directlyencrypted using identity-based encryption, but rather may be encryptedusing a symmetric key, which in turn is encrypted using identity basedencryption and stored in association with the data (e.g., as part of theencrypted data structure that includes the encrypted data). A recipientof the encrypted symmetric key can decrypt the symmetric key and use itto decrypt the content.

In this situation, the bulk of the data payload (e.g., the datastructure video or other content) is encrypted using a symmetric keythat has itself been encrypted using identity-based encryption, ratherthan being encrypted directly using identity-based encryption. Thisgenerally provides a speed advantage over encrypting all of the datadirectly using identity-based encryption without using theembedded-symmetric-key approach, because symmetric keyencryption/decryption is highly efficient. Because either of theseapproaches (direct or indirect identity-based encryption) may be used insystem 10 in substantially the same way, both data encryption approaches(single-step and two-step) will be referred to herein as “identity-basedencryption” for clarity.

In the arrangement of FIG. 1, the data distribution services 18 (based,e.g., on servers) that may be used to assist in the distribution of theencrypted data to users are shown as being separate from data packagingservice 16 and content providers 14, but if desired, content providers14, data packaging service 16, and one or more of the distributionservices 18 may be co-located or their functionality provided using anyother suitable number of computing equipment installations or services.

The distribution services may, if desired, allow a user to browse acontent list and select desired content items for downloading. Contentmay also be distributed automatically using servers 18, according to aschedule, or using any other suitable approach.

Steps involved in using an identity-based cryptographic scheme and theequipment of FIG. 1 to distribute content to user devices 22 are shownin FIGS. 2, 3, 4, and 5. The content that is being distributed may beany digital information (e.g., text, graphics, audio, video, commands,executable code, data, etc.) that is to be conveyed in a secure manner.

The steps of FIG. 2 relate to setting up and using policy enforcementservice 20 in system 10. At step 24 of FIG. 2, policy enforcementservice 20 of FIG. 1 obtains a master secret s. For example, the policyenforcement service 20 may create a master secret from a number that israndomly generated at the service by a processor housed inside atamper-proof enclosure. The master secret may also be produced off-siteand delivered to the policy enforcement service 20. The master secret(also sometimes referred to as a secret master key or a master key) issecret information that will subsequently be used by the policyenforcement service 20 to generate private keys (e.g., private keys sQin systems of the type described in the work of Boneh and Franklin) forusers 22 in the system to use in decrypting encrypted data and togenerate public parameter information (e.g., P and sP) for use by datapackaging service 16 in encrypting data (e.g., the data of datastructures) prior to distribution.

During step 24, the policy enforcement service 20 may generate thepublic parameter information. For example, the policy enforcementservice 20 may obtain or generate a public parameter P. Depending on thespecific type of identity-based encryption being used, other publicparameter information may also be generated. For example, when anidentity-based encryption scheme of the type described in the work ofBoneh and Franklin is involved, the values of the master secret s andpublic parameter P may be used by the policy enforcement service 20 togenerate an additional corresponding public parameter sP. Thus, insystems based on the Boneh and Franklin scheme, the public parameterinformation may include both P and sP.

The parameters P and sP in systems of the type described in the Bonehand Franklin work may be numbers. In general, there is an equivalencybetween numbers, letters, symbols, and other such schemes forrepresenting information. Sometimes certain information (e.g., themaster secret or public parameters) will be described as being in numberform and sometimes certain information (e.g., data attributes that maybe used as policy information) may be described as being at least partlyin character form (e.g., in the form of a rating, etc.). Because of theinherent equivalency between these different representational schemes,the techniques involved in converting letters or symbols into numbers orfor representing multiple numbers or strings as a single number or othersuch operations are not described in detail herein.

At step 24, the policy enforcement service 20 may make the publicparameter information (e.g., parameters P and sP) available to datapackaging services such as data packaging service 16 or any otherservice or entity that is to encrypt content. For example, the publicparameter information may be made available over the communicationsnetwork 12 using computing equipment (e.g., a server) at policyenforcement service 20. The public parameter information may be providedto the data packaging service via courier (e.g., on a diskette or otherstorage media), etc. The public parameter information (e.g., parametersP and sP) may be provided to services such as data packaging service 16as part of a downloadable or preinstalled software module or package.For example, public parameter information (e.g., public parameters P andsP) may be incorporated into or provided with the identity-basedencryption engine software package used by the data packaging service.

Public parameters such as parameters P and sP may be made available tothe data packaging service 16 together or separately. If parameters suchas parameters P and sP are distributed separately, each parameter may bedistributed using a different distribution mechanism. For example, P maybe built into the data packaging service's software and sP may bedistributed over the Internet. Moreover, P and sP may be combined toform the equivalent of a single number or parameter (still referred toherein using the plural form “parameters” for clarity and convenience)or may be subdivided (e.g., to form three or more public parametersub-parts). If desired public parameters P and sP may be provided todata packaging service 16 from policy enforcement service 20 by printedmail. These are merely illustrative examples. Any suitable technique maybe used to make the public parameter information such as publicparameters P and sP available to data packaging service 16 at step 26 ifdesired.

Once the public parameter information (e.g., parameters P and sP) hasbeen provided to the data packaging service 16, the data packagingservice may encrypt data (e.g., data structures containing content thatthe data packaging service obtains from content providers 14). Theencrypted data may be distributed to users 22 (e.g., via datadistribution services 18 and communications network 12). Policyinformation that governs the access rules for accessing the contents ofthe encrypted data may be used to form a public key that is used as ininput when encrypting the data and that may later be used in decidingwho can access the content (i.e., the unencrypted version of theencrypted data). Some policy information used in system 10 may beglobally known (e.g., rated-R movies require users to be 17 years of ageor older). Other policy information may not be globally known (e.g., theprice for this movie is $10.00 or rated G movies can be viewed by peopleof all ages). The policy information (e.g., the policy information thatis in the public key and that is not globally known) may be conveyed tothe users 22 and policy enforcement service 20 over communicationsnetwork 12.

A user who desires to access a particular encrypted data item maygenerate an access request for the policy enforcement service. Theaccess request directs the policy enforcement service to provide theuser with a private key (if the user is authorized) that corresponds tothe public key that was used to encrypt that data item. The accessrequest may include the public key and the policy informationrepresented by that public key (or may include information identifyingthe relevant public key to the policy enforcement service) and this maybe provided to the policy enforcement service 20 over communicationsnetwork 12.

At step 28, the policy enforcement service may receive an access requestfrom a user who desires to access the content of a particular piece ofencrypted data that the user has received from the data packagingservice 16. The access request is in effect a request from the user fora copy of the private key that corresponds to the public key that theuser is providing or identifying through the access request and that wasused by the data packaging service's identity-based encryption engine toencrypt the data structure.

At step 30, the policy enforcement service 20 may generate the privatekey (sQ) using the policy enforcement service's knowledge of the mastersecret s and the public key (policy information) Q and using theidentity-based encryption algorithm. During step 30, the policyenforcement service may use the policy information of the public key todetermine whether or not the requesting user is entitled to receive theprivate key. For example, if the public key (policy information) is arating (e.g., rating=R), the policy enforcement service 20 may attemptto ascertain whether or not the user is authorized to view R-ratedcontent before providing the private key to the user. The policyenforcement server may, for example, ascertain the user's age andcompare the user's age to the age associated with R-rated content (theage information may be part of the policy information in the public keyor may be related global policy information that is already known by thepolicy enforcement service). Global policy information that ismaintained at the policy enforcement service may be used by the policyenforcement service to supplement or override the policy information setforth in the public key (e.g., additional constraints may be added). Alook-up table or other suitable database arrangement may be used by thepolicy enforcement service to allow the policy enforcement service toidentify an appropriate public key and corresponding policy (and tothereby identify the appropriate private key being requested) when anaccess request does not explicitly include all of the necessary policyinformation, but rather provides sufficient information for the policyenforcement service to identify the relevant policy.

During the verification process of step 30, the policy enforcementservice 20 may need to ascertain certain information about the user(e.g., age, subscription plan, location, etc.). The user information maybe represented by any suitable strings, numbers, symbols, etc. Privatekeys may be made to automatically expire in system 10 by automaticallyconcatenating the current time (e.g., the current day of the year andyear, the current month, or any other suitable time-related date-stampinformation) with the public key used during encryption at datapackaging service 16. For clarity, the public key is generally describedherein by the value Q. A suitable mathematical function may be used todetermine the value of Q suitable for use as an input to theidentity-based encryption private key generation algorithm used atpolicy enforcement service 20 from a string representation of the publickey (policy information) or any other suitable representation.

Any suitable manual or automatic authentication technique may be used bythe policy enforcement service 20 when verifying the user's accesspermission at step 30. For example, the user may be asked to fax or maila letter containing user information to the private key generator 16 onthe user's official letterhead, which is examined for authenticity bypersonnel or automated equipment at the private key generator. Asanother example, biometric identification techniques (e.g., fingerprintanalysis, eye-scanning, handprint or voiceprint analysis, facialrecognition methods, or in-person identification checks) may be used.When the authentication process involves electronic communicationsbetween the user and the policy enforcement service 20, thecommunications path between the user and the policy enforcement service20 should be secure. The communications path may be assumed to be secureif it is not available for use by untrusted parties. For example, thenetwork between the policy enforcement service 20 and the user 22 may bea private line that is controlled by the policy enforcement service 20or another trusted authority. As another example, a secure channel maybe supported using a secure web browser link (e.g., using the securesockets layer protocol). After appropriate information has been gatheredabout the user and after the policy enforcement service 20 has appliedthe appropriate policy rules (from the public key policy informationassociated with the encrypted data) to verify the user's authorizationto receive the private key at step 30, the private key may be providedto the user at step 32 (e.g., using a secure path such as a securesockets layer (SSL) link over network 12). This is merely anillustrative technique for providing the private key to the user. Anysuitable technique may be used if desired.

After the policy enforcement service 20 provides the private key to thereceiver at step 26, processing may continue at step 28 (as indicated byline 34), so that the policy enforcement service 20 may receiveadditional requests for private keys from other users based on thepublic keys they provide.

Steps involved in using the data packaging service 16 in system 10 areshown in FIG. 3. At step 36, the data packaging service 16 may obtainthe identity-based encryption public parameter information (e.g., publicparameters P and sP). This information may be supplied by policyenforcement service 20. The public parameter information may be suppliedusing any suitable technique. For example, service 16 may be providedwith the public parameter information electronically over network 12 orthe public parameter information may be provided as part of theidentity-based encryption engine software.

At step 38, the data packaging service 16 may obtain the data that is tobe encrypted. Any suitable content may be distributed using system 10,such as media, software, text, graphics, financial records or otherdata. As an example, a digital version of a movie may be provided todata packaging service 16 using a satellite uplink (part of network 12)or by providing a CD or DVD containing the movie file to the service 16.

At step 40, the data packaging service 16 may, if desired, form a datastructure of a particular type (e.g., the type “movie”). The content ofthe movie may be placed in the data structure. Attributes may beassociated with the data. Such attributes may, for example, definepolicy information. As an example, the attributes of a movie may includethe movie's title, director, running time, year of release, rating(e.g., G, PG, PG-13, R, X, etc.), price, etc. The attributes associatedwith the data may (at least initially) be maintained separate from thedata (e.g., as when a rating label is affixed to the outside of a VCRtape containing a movie) or may be placed in a data structure along withthe content to be encrypted (e.g., when track, artist, title, and otherattributes are placed in an MP3 data structure with the data for asong).

Some or all of the attributes and other suitable information governingthe use of the data may be used as policy information. This policyinformation may be used by the data packaging service in constructingthe public key Q for encrypting the data structure. An example of asuitable public key for a movie may be the movie's rating (e.g.,rating=R). If desired, more policy-related attributes may be used toform the public key. As an example, a number of attributes for a digitalvideo may be concatenated together to form the public key Q (e.g.,attributes such as “secure-video://Name=Matrix;Distributor=Paramount;Date=Aug-2002;Rating=“R.” If desired, the data tobe encrypted may be placed in a data structure such as an XML datastructure. The policy data (attributes) that are used by the datapackaging service 16 as the public key Q may be manually selected or maybe automatically selected. If desired, data type encryption policyinformation may be used to define which attributes and policyinformation are used when forming Q. One set of attributes may be usedfor encrypting data of one type (e.g., when encrypting data in datastructures of the type movie), whereas another set of attributes may beused for encrypting other types of data (e.g., when encrypting data indata structures of the type song). The data type encryption policyinformation may be provided in an XML format if desired.

The data may be encrypted at step 40 using the identity-based encryptionengine at the data packaging service 16. The identity-based encryptionengine implements the identity-based encryption algorithm and convertsunencrypted information into encrypted information based on publicparameter information and identity information (called the public key inthe context of system 10 of FIG. 1). In particular, the identity-basedencryption engine may use the identity-based encryption public key Q andthe identity-based encryption public parameter information as first andsecond inputs to the identity-based encryption engine in addition to thedata being encrypted.

The resulting encrypted data (and optionally the public key Q and othersuitable policy data if desired) may be made available to users at step42. For example, the encrypted data structures may be distributed touser equipment 22 via communications network 12 using distributionservices 18. Storage media such as CDs and DVDs may also be used todistribute the encrypted data structures. Because the data is encrypted,peer-to-peer distribution processes may be used (e.g., one user 22 mayemail or otherwise transmit a received encrypted data structure or dataitem to another user 22 via communications network 12). Users 22 maydownload encrypted data (e.g., from a repository provided on adistribution service 18). Users may also receive the encrypted data whenthey originally receive the user device 22 (e.g., if the encrypted datais already stored on a hard drive in the user device 22 when the userreceives the user device). Once the encrypted data has been madeavailable to users at step 42, additional content may be obtained atstep 38 (i.e., the process may continue), as indicated by line 44.

Illustrative steps involved in allowing a user to obtain access to thecontent (unencrypted version) of the encrypted data are shown in FIG. 4.The system may be set up at step 46. For example, the software that theuser's equipment may use to perform access functions and associatedparameters may be obtained at step 46. The necessary software may bepreinstalled on the user's equipment, may be downloaded, or may beprovided via a diskette, CD, or other storage media. The software may bea software package provided by a particular institution (e.g., a bank)or may be software provided by a distributor or manufacturer ofgeneral-purpose software (e.g., a provider of media player software). Ifthe encrypted data is a digital movie file, for example, the softwaremay be video software or media player software provided by a partyassociated with a digital subscription movie service. The video softwaremay be used to handle the process of selecting a desired movie titlefrom among various titles listed in a list provided by distributionserver 18. The video software may be used to download the encryptedversion of the movie. The video software may then be used by the user tohandle the process of generating an access request (incorporating theappropriate public key), receiving the appropriate corresponding privatekey from the policy enforcement service, and decrypting and playing themovie for the user.

After the system has been set up at step 46, the user may obtain theencrypted data that is to be accessed. The user may, for example, obtainthe encrypted data from the data packing service 16 via one or more ofdistribution services 18 or other suitable equipment. The encrypted datamay also be prestored on the user's hard drive, in which case theprocess of obtaining the encrypted data may involve a local disk accessfunction. Policy information (e.g., information such as included in thepublic key Q) may be provided to the user at the same time (or adifferent time) as the encrypted data or may be pre-stored on the user'sequipment (e.g., in a table that the user accesses when it is time toprovide the private key to the policy enforcement service). This maymake the authentication and key generation process easier, because theuser may simply forward the relevant policy information to the policyenforcement service 20 as part of the key request.

At step 50, the user may manually or automatically (using theappropriate user software such as the video playback software) generatea key request (i.e., the user may ask the policy enforcement service 20to generate and provide the user with an appropriate private key todecrypt the encrypted data structure). The key request, which mayinclude all of the public key/policy information except pre-storedglobal policy information (so-called because it may affect all users insystem 10) or which may otherwise include appropriate information sothat the policy enforcement service can determine which policy to applyand which private key to generate may be transmitted to the policyenforcement service 20 (e.g., over network 12). During the key requestprocess, the policy enforcement service 20 uses the appropriate policyinformation for the data (i.e., the policy information associated withthe data that the user has identified or has actually forwarded to thepolicy enforcement service and/or global or pre-stored policyinformation, etc.) to determine whether or not to grant the user accessto the content. As an example, the policy information may containinformation that specifies an allowable age range for users. The policyinformation may be, for example, a movie rating R, that dictates thatonly users of age 17 and greater may access the content of theassociated encrypted data. As another example, all users with a certainsubscription type (e.g., the “silver plan”) may be granted access. Theseare merely illustrative policy types that may be used. Different andmore complex policies may be used if desired. Moreover, the requestprocess of step 50 may require that the policy enforcement serviceauthenticate the identity of the user (or at least user characteristicssuch as age, etc.). Any suitable verification techniques may be used ifdesired. Moreover, the user may be asked to provide additionalinformation during the user's interactions with the policy enforcementservice. Such additional information may include, for example, paymentinformation such as the user's credit card number, etc. (as required,e.g., by the policy information).

If the user provides appropriate information to the policy enforcementservice 20 and if the policy enforcement service uses the policies setforth in the policy information and the user information to successfullyverify that the user is authorized to access the encrypted data, thepolicy enforcement service 20 may issue a private key to the user andthe user may obtain this key at step 52. The private key (sQ)corresponds to the public key Q that was used to encrypt the data andthat was identified by or provided with the access request. Accordingly,the user may, at step 52, use the private key sQ to decrypt theencrypted data and to access and use the content of the data. In theillustrative situation of the encrypted movie, for example, the user maydecrypt and play back the movie with the user's video software. Thedetails of the key request process can be hidden from the user, so thatthe user need not ever know that a key is being requested by the user'ssoftware. As shown by line 54, the user may continue to obtainadditional content by repeating the processes starting with step 48.

The data that is distributed using system 10 may, in general, be in anyformat. For example, digital movies may be stored as MPEG files, songsmay be stored as MP3 files, etc. If desired, the data may be stored indata structures such as data structures using an XML format. (The datacontent may still be in an MPEG or MP3 file format.)

Particular formatting information may be used to determine which of thedata attributes (e.g., a rating or some other attribute or a particularcombination of attributes) are to be used in forming the public key Qthat is subsequently used to encrypt the data. This so-called data typeencryption policy information may be specific to the particular datatype involved. For example, the data type encryption policy informationmay be used to dictate that all movies be encrypted using one set ofpolicy information and that all songs be encrypted using another set ofpolicy information. If desired, the data type encryption policyinformation may itself be provided in XML format. An illustrativescenario in which both the data and the data type encryption policyinformation are provided using XML is described above in connection withFIGS. 7 a, 7 b, and 7 c.

Illustrative steps involved in using XML data type encryption policyinformation to determine how to encrypt data are shown in FIG. 5. Atstep 56, the identity-based encryption engine at data packaging service16 may obtain and/or create the data (e.g., data in data structures) tobe encrypted. The data may contain content (e.g., a movie or song), maycontain data type information (e.g., “movie” or “song”), and may containor otherwise have other associated attributes (e.g., an attribute named“rating” with its corresponding attribute value of “R”—selected frompossible values of G, PG, R, and X). Content is generally provided bycontent providers 14. Other information for the data (e.g., data typeand other attribute information) may be provided by the data packager,the content provider, or other suitable entities or combinations of suchentities.

At step 58, the data packaging service 16 may obtain data typeencryption policy information. This information may be obtained from thecontent provider, may be created by personnel at the data packagingservice 16, or may be created by any other suitable entity. The datastructure type encryption policy information may, for example, specifythat for the data type “movie,” the attribute “rating” is the soleattribute to be used in forming the public key Q. As another example,the data type encryption policy information may specify that for thedata type “song” the two attributes “artist” and “price” are to beconcatenated together to form Q. These are merely illustrative examplesof data type encryption policy information. Any suitable data typeencryption policy information may be used if desired.

The data type encryption policy information may be provided andmaintained in XML format or any other suitable form.

At step 60, the data packaging service 16 may use the data typeencryption policy information and the attribute information associatedwith the data to form the public key Q. The data packaging service maythen use the identity-based encryption engine to encrypt the data andits associated content. The identity-based encryption engine may use asinputs the public parameter information provided from the policyenforcement service 20 (P, sP) and the public key formed from the policyinformation (Q).

The foregoing is merely illustrative of the principles of this inventionand various modifications can be made by those skilled in the artwithout departing from the scope and spirit of the invention.

1. A method for using identity-based encryption (IBE) to distribute datato users in a system that has a content provider, a data packagingservice, a policy enforcement service, a plurality of users atrespective user devices, and a communications network, comprising:providing a data structure from the content provider to the datapackaging service over the communications network, wherein the datastructure includes data to be encrypted and data attributes for thatdata; at the data packaging service, using at least some of the dataattributes from the content provider as policy information to form anidentity-based-encryption public key that is not specific to a singleuser, wherein the data packaging service uses data type encryptionpolicy information to select which of the data attributes are used whenforming the identity-based-encryption public key; at the data packagingservice, encrypting the data using an identity-based encryption engine,wherein the identity-based encryption engine uses theidentity-based-encryption public key and identity-based-encryptionpublic parameter information as inputs when encrypting the data; makingthe encrypted data available to the plurality of users at the userdevices; at a given one of the users, generating a key request for anidentity-based-encryption private key corresponding to theidentity-based-encryption public key, wherein the key request includesthe policy information; receiving the key request from the given user atthe policy enforcement service; at the policy enforcement service, usingthe policy information from the key request to determine which accesspolicy applies to the given user and to determine whichidentity-based-encryption private key to generate in response to the keyrequest; using the policy information from the key request andinformation on at least one characteristic of the given user at thepolicy enforcement service to determine whether the given user isauthorized to receive the requested identity-based-encryption privatekey; and if the given user is authorized, using the policy enforcementservice to transmit the requested identity-based-encryption private keyto the user that corresponds to the identity-based-encryption publickey.
 2. The method defined in claim 1 further comprising using theprivate key and a decryption engine at the user device of the given userto decrypt the encrypted data so that the given user may access thecontent of the encrypted data.
 3. The method defined in claim 1 whereinthe policy information includes rating information, the method furthercomprising using the rating information in determining whether the givenuser is authorized to receive the requested private key.
 4. The methoddefined in claim 1 wherein the policy information includes ratinginformation selected from the group consisting of: a G rating, a PGrating, a PG-13 rating, an R rating, and an X rating.
 5. The methoddefined in claim 1 wherein a media player is implemented on the userdevice of the given user, the method further comprising generating thekey request using the media player.
 6. The method defined in claim 1further comprising using the identity-based encryption engine toautomatically encrypt the data based on data type encryption policyinformation that is in an XML format.
 7. The method defined in claim 6,wherein the data is in an XML format, the method further comprisingusing the identity-based encryption engine to automatically encrypt theXML-format data based on the XML-format data type encryption policyinformation.
 8. The method defined in claim 1 wherein the public keycomprises an XML-format public key and wherein using the policyinformation of the public key and information on at least onecharacteristic of the given user at the policy enforcement service todetermine whether the given user is authorized to receive the requestedidentity-based-encryption private key further comprises using theXML-format public key in determining whether the given user isauthorized to receive the private key corresponding to the public key.9. The method defined in claim 1 wherein the data comprises a song, themethod further comprising distributing the song to users using apeer-to-peer arrangement in which one user electronically transmits thesong to another before the data is decrypted.
 10. The method defined inclaim 1 further comprising distributing the encrypted data from one userto another in a peer-to-peer arrangement before decrypting the encrypteddata.
 11. The method defined in claim 1 further comprising encryptingthe data directly using an identity-based encryption algorithm withoutusing symmetric key encryption to encrypt content in the data.
 12. Themethod defined in claim 1 further comprising encrypting the data using atwo-step process in which content in the data is encrypted usingsymmetric key and the symmetric key is encrypted using an identity-basedencryption algorithm.
 13. The method defined in claim 1 furthercomprising transmitting the public key from the data packaging serviceto the users over the communications network.
 14. The method defined inclaim 1 further comprising using global policy information at the policyenforcement service in determining whether the given user is authorizedto receive the requested private key.
 15. The method defined in claim 1wherein the information on the characteristic of the given user includesinformation on the given user's age, the method further comprising usingthe policy information from the key request and the information on theuser's age at the policy enforcement service to determine whether thegiven user is authorized to receive the private key corresponding to thepublic key.